A natural hollow in fibrous tissues, lined by smooth cells and containing a little fluid. It is found where there is much pressure or friction, and its purpose is to allow free movement without stretching or straining the tissues: for example, on the knee-cap or the point of the elbow, and, generally speaking, where one muscle rubs against another or against a bone. A bursa may form beneath corns and bunions, or where a bone presses on the skin.